[Patricia A]

Maybe a vet visit to make sure there is she doesn’t have an abscessed anal gland. .Very painful for her. . The change in diet might have led to constipation and she was unable to express her anal glands with bowel movements. Hence the impaction of glands.
Here’s a site which explains better then me. Hope it helps.
https://pets.webmd.com/dogs/anal-sac-disease-dogs